It is given that the derivative of a function that is the sum of two other functions, is equal to the sum of their derivatives. This can be proved by using the derivative by definition or first principle method.

Consider a function of the form .

First we take the increment or small change in the function.

Putting the value of function in the above equation, we get

Dividing both sides by , we get

Taking the limit of both sides as , we have

By the definition of derivative we have

This shows that the derivative of the sum of two given functions is equal to the sum of their derivatives.